The present invention relates to a wavelength conversion device in which a laser beam is applied on a nonlinear optical medium so as to obtain a beam of a different wavelength.
In the prior art, the laser generates a coherent beam of sharp directivity and is widely applied in various fields such as material processing and measurement fields. Recently, it has also been applied in medical and chemical industry fields. However, with the exception of some types, the laser vibrates or oscillates only at a specific wavelength which results in one of the most serious obstacles to its applications. Wavelength conversion techniques using various kinds of nonlinear optical materials (which are also used in the present invention) are known in the prior art. The principles of the prior art techniques will be explained in the following paragraphs.
In such prior art approaches, when two laser beams having different frequencies enter a nonlinear optical crystal, a polarized wave having an intensity which is proportional to the product of the electric fields of these two laser beams is generally generated in the interior of the crystal in addition to polarized waves having intensities which are proportional to the respective laser beams. Since a polarized wave having an intensity which is proportional to the product of the electric fields has a frequency corresponding to the sum of the frequencies of both laser beams, it is possible, by extracting this polarized wave, to convert the laser beam into a beam having a frequency or a wavelength different from that of the original beam. This is the principle of wavelength conversion which is called frequency sum generation. A known form of wavelength conversion called second harmonic generation (hereinunder referred to as "SHG") is particularly important in practical use. This is a system for converting two laser beams having the same frequency, namely a single laser beam, into a beam having twice the frequency or half the wavelength of the original beam, by projecting the laser beam onto a nonlinear optical crystal.
The concept of converse wavelength conversion, namely, converting a laser beam into two laser beams having the frequencies the sum of which is equal to the frequency of the original beam by projecting the laser beam onto a nonlinear optical crystal is also known. In this case, the nonlinear optical crystal is generally incorporated into a resonator and both or either of the two converted beams is extracted by oscillation under a controlled oscillating condition. This is called an optical parametric oscillation (hereinunder referred to as "OPO") system.
In both SHG and OPO systems, it is necessary that the laser beam and the converted beam travel in phase with each other in order to enhance the wavelength conversion efficiency. This is called phase matching. For example, in the SHG system the fundamental wave, which is a polarized wave having the same frequency as the incident laser beam, and a second harmonic wave, which is a polarized wave having twice as large a frequency as the incident laser beam, are caused to travel in the same direction with the phases matched with each other. For this purpose, the velocities of both waves and, hence, the refractive indexes of the nonlinear optical crystal with respect to those waves must be the same. Ordinarily, however, the higher the frequency of light, the higher the refractive index, so that the phase matching condition is generally not able to be satisfied.
There is also known a phase matching method utilizing the optical anisotropy of a crystal. Light is split into an ordinary ray and an extraordinary ray when travelling in an optically anisotropical crystal, and the refractive index of the ordinary ray does not depend upon the direction of travel but the refractive index of the extraordinary ray depends upon the direction of travel. This phenomenon is utilized for selecting the angle of incidence of a laser beam so that the refractive indexes of the laser beam with respect to the fundamental wave and the second harmonic wave are the same. This method is called an angle phase matching method.
This method, however, has disadvantages in that the phase matching condition is influenced by the double refraction of an optical crystal. To solve this problem, the angle of incidence of a laser beam with respect to a nonlinear optical crystal is fixed at 90.degree. with respect to the optical axis so as to prevent the generation of double refraction, and the phase is matched by utilizing the temperature dependence of the refractive indexes of an ordinary ray and an extraordinary ray by adjusting the temperature of the nonlinear optical crystal. This method is called temperature phase matching.
In these conventional methods, there are two types of phase matching conditions. In a phase matching condition of type I, the fundamental wave is assumed as an ordinary ray while the second harmonic wave is assumed as an extraordinary ray. On the other hand, in a phase matching condition of type II, the fundamental wave is assumed as an extraordinary ray while the second harmonic wave is assumed as an ordinary ray. Note that hereinafter "ordinary" and "extraordinary" rays are also referred to as "normal" and "abnormal" rays, respectively.
Details of the theoretical basis of wavelength conversion and phase matching as described above, as well as certain other prior art techniques, will be described in the following section.